1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a facsimile, and a copying machine.
2. Description of the Related Art
Conventionally, an image forming apparatus is well-known where a plurality of image forming sections that corresponds to a plurality of colors such as black and forms images of the corresponding colors on their own image carriers are disposed (see Japanese Patent Application Laid-open No. 2006-201743 or the like).
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743, there are a direct transfer position where a black color image formed by the black color image forming section is directly transferred to a recording medium and a secondary transfer position where other color images primarily transferred onto an intermediate transfer belt by remaining other color image forming sections are secondarily transferred from the intermediate transfer belt to the recording medium. The secondary transfer position is disposed at an upstream of the direct transfer position in a recording medium transport direction. The intermediate transfer belt is rotatably suspended by a plurality of roller members, so that the intermediate transfer belt is rotated by a driving roller which is one of the roller members. In addition, a recording medium transport belt which is rotatably suspended by a plurality of roller members is disposed to transport the recording medium mounted thereon so as to pass through the direct transfer position and the secondary transfer position. In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743, the recording medium is allowed to pass through the direct transfer position and the secondary transfer position by the recording medium transport belt, so that a full color image is formed on the recording medium by superimposing other color images transferred from the secondary transfer position to the recording medium and the black color image transferred from the direct transfer position to the recording medium. In addition, since the recording medium is mounted on and transported by the recording medium transport belt, a change of the recording medium transport path between the direct transfer position and the secondary transfer position is suppressed, so that it is possible to stably transport the recording medium between the direct transfer position and the secondary transfer position.
In the image forming apparatus disclosed in Japanese Patent Application Laid-open No. 2006-201743, since the images are transferred at the direct transfer position or the secondary transfer position to the recording medium mounted on and transported by the recording medium transport belt, due to the difference in the transfer positions of the images, color irregularity is likely to occur in the images transferred to the recording medium.
Therefore, a technology is considered where a pattern image of each color is formed on the recording medium transport belt, the pattern image is sensed by an optical sensor which is disposed at a downstream of the direct transfer position and the secondary transfer position in the rotation direction of the recording medium transport belt, and correction of an image formation condition is performed based on a sensing result so as to reduce the color irregularity.
However, due to the periodic change in speed with a period of one rotation of a roller member caused by eccentricity, load change, or the like of the roller member, by which the recording medium transport belt is rotatably suspended, the periodic speed change of the rotation speed of the recording medium transport belt occurs with a period of one rotation of the roller member. If the phases of the speed change of the recording medium transport belt are different between at the direct transfer position and at the sensing position where the pattern image is sensed by the optical sensor, the black pattern image which is directly transferred at the direct transfer position to the recording medium transport belt is erroneously sensed so that the pattern image is expanded or contracted at the sensing position than when directly transferred. Similarly, if the phases of the speed change of the recording medium transport belt are different between the secondary transfer position and the sensing position, the other color pattern images which are secondarily transferred at the secondary transfer position to the recording medium transport belt are erroneously sensed so that the pattern images are expanded or contracted at the sensing position than when secondarily transferred. Therefore, there is a problem in that accuracy of the correction deteriorates due to the correction performed based on the sensing result which is obtained by the erroneous sensing.
In addition, a technology is considered where a pattern image of each color is formed on the intermediate transfer belt, the pattern image is sensed by an optical sensor which is disposed at a downstream of the secondary transfer position in an intermediate transfer belt rotation direction, and correction of an image formation condition is performed based on a sensing result so as to reduce the color irregularity.
However, due to the periodic speed change with a period of one rotation of the roller member caused by eccentricity, load change, or the like of the roller member, by which the intermediate transfer belt is rotatably suspended, the periodic speed change of the rotation speed of the intermediate transport belt occurs with a period of one rotation of the roller member. If the phases of the speed change of the intermediate transfer belt are different between at the secondary transfer position and at the sensing position where the pattern image is sensed by the optical sensor, the black pattern image which is reversely transferred at the secondary transfer position onto the intermediate transfer belt is erroneously sensed so that the pattern image is expanded or contracted at the sensing position than when reversely transferred. Similarly, if the phases of the speed change of the intermediate transfer belt are different between at the primary transfer position and at the sensing position, the other color pattern images which are primarily transferred at the primary transfer position to the intermediate transfer belt are erroneously sensed so that the pattern images are expanded or contracted at the sensing position than when primarily transferred. Therefore, there is a problem in that accuracy of the correction deteriorates due to the correction performed based on the sensing result which is obtained by the erroneous sensing.